Conventionally, a centrifugal pump has an impeller and a casing. An actuator such as an electric motor operates the impeller rotatably, and the casing houses the impeller. The impeller has blades arranged one after another in a circumferential direction, and the blades define a passage for pressurizing fluid. Specifically, in a low-specific-speed centrifugal pump, it is well-known that the passage is curved spirally toward an outer peripheral side, and a cross section of the passage decreases toward the outer peripheral side (refer JP 2002-122095 A and JP 2005-023794 A, for example).
That is, according to such a centrifugal pump, fluid flows out of the impeller after passing through the passage while velocity energy of the fluid is changed into pressure energy. Accordingly, the centrifugal pump may have a preferable configuration as a low-specific-speed type since the fluid can be pressurized with less friction loss.
However, since the passage is curved, a passage length from an inlet to an outlet becomes long, and a swirl flow can be caused easily by a separated flow. The swirl flow may cause noises. In addition, saving energy is a trend in these days, and it is required that a torque applied to the impeller is reduced and that a rotation speed of the impeller increases. As a result, the separated flow may be occurred easily and noise can be caused easily.